An extrusion head of the above-described general type is disclosed in German Patent Specification No. 3 212 157 and in British Patent Specification No. 2 131 734. In such prior specifications, the manufacture of a profile made of rubber, which is ultimately intended for use in the production of a motor vehicle tire or the like is discussed.
A rubber mixture is extruded in an extrusion device. The mixture is forced into a flow channel in the extrusion head by the extrusion device and is shaped by the flow channel and by the extrusion nozzle to produce a profile of the desired configuration.
If the material being extruded is a highly viscous mixture of rubber and/or plastics materials, particularly if the extruder screw or screws utilised have a relatively large diameter, a problem arises during the commencement of the extrusion process in that an air-filled cavity is formed immediately downstream of the end of the extrusion screw or screws. The air is gradually removed from the cavity by being incorporated in the mixture or by being entrained therewith. In consequence, air bubbles will be present in the extruded profile until substantially all the air, which collects downstream of the end of the extrusion screw when the extrusion head commences its operation, has been discharged from the flow channel portion. In a relatively large extrusion device, this disadvantage means that, for a period of approximately two hours, a defective profile is produced and only thereafter is a profile produced which is free from air bubbles.